The quarterbacks are ranked by their rating compared to the rest of the league; 100 is average, 115 or so is Pro Bowl Level and above 130 is an all-time great season. QB Rating is not the perfect measure of a quarterback, but it is probably the best available. Joe Montana has 3 of the top 6 seasons; he and Steve Young are curious cases because I can’t separate their contribution from the system they played in. By the way, they are probably my two favorite QBs of all time.